This phonetic map is quite similar to the way the Gtk cyrillic input module works: for your convenience, some symbols can be entered in different ways; for instance, to enter the "shcha" you can type "sj" (as in the gtk module) or "sht".

install details

Just copy the file to $VIM/vimfiles/keymap; to enter cyrillic mode, issue the command:
:set keymap=bulgarian